Air assistance apparatus for computing the airflow provided by only means of pressure sensors

ABSTRACT

An apparatus ( 1 ) to assist a patient&#39;s respiration by delivering air to a patient through a mask, the mask designed to be connected to a first extremity of a tube, the apparatus including: a control unit ( 2 ) to adjust the pressure delivered by a blower ( 4 ), a first pressure sensor ( 6 ) for sensing pressure PM at the first tube extremity and being connected to the control unit, and a second pressure sensor( 8 ) for sensing pressure PB at the air output of the blower and being connected to the control unit; so that, when a tube is connected to the mask and connected to the apparatus at its second extremity, the air flowing from the apparatus to the mask, the control unit is able to calculate the airflow at the tube&#39;s second extremity from the pressures PM and PB and from the tube&#39;s airflow resistance coefficient K t .

This application is a U.S. National Stage of International applicationPCT/IB03/01403, filed Mar. 10, 2003 and claims benefit of U.S.provisional application Ser. No. 60/362,441, filed Mar. 8, 2002.

TECHNICAL FIELD

This invention concerns the field of apparatus to assist a patientrespiration and more specifically an apparatus able to calibrate thetubes connected to the patient's mask and to the blower and to determinethe airflow of the patient's from pressure measurement and from the tubecalibration.

BACKGROUND ART

In many treatments apparatus are used to provide patients with air. Morefrequently it is used for patients with a breathing deficiency causedfor example by the weakness of the breathing system or by obstructiveapneas during the sleep. In those cases it is important to control thepressure of the air delivered to the patient. With respiratoryinsufficient patients, apparatus providing air at a higher pressure helpto compensate the weakness of the patients lungs. In the case ofpatients suffering of sleep apneas, providing the air at a higherpressure removes the obstruction of the upper airways.

In order to provide a correct treatment, it is required to accuratelyknow the value of airflow the patient is provided with. Usually theapparatus determine the airflow by measuring airflow in the patientcircuit (between the blower and the mask) using an airflow sensor.Airflow sensors can be based on high sensitivity differential pressuresensors measuring the pressure drop across a low resistance or usingpitot tubes, or hot wire airflow sensors. Another commonly used mean toevaluate the airflow is by measuring the blower parameters such asspeed, current consumption and power.

SUMMARY OF THE INVENTION

The first object of the invention is to determine the airflow at thepatient's mask, without using sensors like airflow sensors that areexpensive or motor consumption or rotation sensors that are not accurateespecially at low flows.

The invention thus concerns to assist a patient respiration bydelivering air to a patient through a mask, said mask being designed tobe connected on one first extremity of a tube, said apparatuscomprising:

-   -   a control unit to adjust the pressure delivered by the blower of        said apparatus,    -   a first pressure sensor for sensing the pressure PM at the first        tube extremity and being connected to the control unit, and    -   a second pressure sensor for sensing the pressure PB at the air        output of said blower and being connected to said control unit.        These elements are comprised in the apparatus in order that,        when a tube is connected to the mask and connected to said        apparatus on its second extremity, the air flowing from the        apparatus to the mask, said control unit is able to calculate        the airflow at said second extremity of the tube from the        pressures PM and PB and from the airflow resistance coefficient        K_(T) of the tube

An other implementation of the apparatus according to the invention isto provide an apparatus being able to calculate the coefficient K_(T) byusing a shell with a traversing hole having a known airflow resistancecoefficient K_(s). This enables to use tubes of different sizes, andeven tubes with different standards of airflow resistance coefficients.

A further implementation of the apparatus according to the invention isthat the pressure control unit comprises an estimation module connectedto the means for detecting the patient's breathing parameters, in orderthat the estimation module is able to determine when the patient isinspiring or expiring and in response the pressure to apply to thepatient's mask, so that the control unit adjusts the pressure deliveredby the blower.

Further implementations enable modulating the pressure of the providedair in response to the patient's breathing parameters and events whichoccur in the patient's breathing.

BRIEF DESCRIPTION OF FIGURES

The purposes, objects and characteristics of the invention will becomemore apparent from the following description when taken in conjunctionwith the accompanying drawings in which:

FIG. 1 represents the apparatus according to the present invention whenused according to a preferential implementation,

FIG. 2 represents the flowchart of the calibration of a tube by theapparatus,

FIG. 3 represents a device of the apparatus according to presentinvention,

FIG. 4 represents the electric schema of the device represented in FIG.3,

FIG. 5 represents an implementation of the apparatus,

FIG. 6 represents the way the apparatus reacts to events occurring inpatient's breathing, FIG. 6( b) represents the auto-adjustment ofExample 2,

FIG. 7 represents how the apparatus operates to detect the presence of apatient at the mask, and

FIG. 8 represents the relationship between Frequency Shift Keying (FSK)modulator and the voltage controlled current source.

DETAILED DESCRIPTION OF THE INVENTION

Ordinary tubes used in an air assisting apparatus usually comprise apressure sensing tube to measure the pressure at the end of the tube.The apparatus according to the present invention is based on thischaracteristic. As represented on FIG. 1, the apparatus 1 is connectedto the tube 20 by the air inlet and connected to the pressure sensingtube of the tube. The pressure sensing tube is connected to a firstpressure sensor 6 comprised in the apparatus. If no sensing tube iscomprised, a mean will be connected at a first pressure sensor of theapparatus. Because the tube 20 diameter value is much smaller than thetube 20 length value, the pressure drop in the tube is defined by thefollowing equation:Δp=K _(T)·airflow²

wherein:

K_(T) represents a constant coefficient characteristic of the tube,

Δp represents the difference of pressure between the two tubeextremities, and airflow is the volume of air per time crossing thetube.

The apparatus has sensors that measure the pressure PB at the apparatusair outlet. As the apparatus is sensing pressure on both sides of thetube, knowing the tube coefficient k is allowing the system to computethe airflow by measuring the pressure drop. The present implementationconsists in operating in a mode where precise airflow is required. Ashell 10 which is a cap which comprises a small hole 12 at his top isplaced to close one extremity of the tube, the other extremity of thetube being connected to the air outlet of the blower 4 of the apparatus1. This calibration shell 10 has an airflow resistance coefficient K_(S)which is a characteristic of it.

Considering that PB being the pressure sensed at the output of theapparatus and PM being the pressure sensed at the calibratedtermination, S0 being known, we have

${Airflow} = {K_{S} \cdot \sqrt{PM}}$${{as}\mspace{14mu}\Delta\; p} = {{{K_{T} \cdot {airflow}^{2}}\mspace{14mu}{then}\mspace{14mu} K_{T}} = \frac{\left( {{PB} - {PM}} \right)}{\left( {K_{S}^{2} \cdot {PM}} \right)}}$

The apparatus is thus able to determinate the tube coefficient K_(T).

The calibration process takes advantages of having a number ofmeasurements made at different pressures levels and by averaging themcan get a more accurate K_(T) value.

Example of a 1.8 m tubing Ø15 mm:

The calibration termination has a S0 coefficient of 18 (airflow unitsare in LPM)

PB is measured at 9.90 hPa

PM is measured at 7.72 hPa

This is giving k=0.000871

Meaning then that the airflow is: 50.02 LPM

When using a new tube, the calibration process is entered, notably atthe request of a clinician or a qualified user. The apparatus isexpecting the calibrating shell to be hooked as described in FIG. 1. Theflowchart represented in FIG. 2 is showing an example of a series ofmeasurements resulting in the K_(T) coefficient calculation, then thetube coefficient K_(T) is recorded and all the upcoming sessions will bebased on this new tube coefficient. When the tube 20 and the calibratingshell 10 are installed, the value J corresponding to the number of onemeasure is set equal to 1 and associated to a number I corresponding tothe value of the pressure provided by the blower 4, this I number beingfor example equal to 4 hPa when the calibration starts. Then the PulseWidth Modulation (PWM) 31 voltage is applied in order to deliver thepressure PM sensed at the calibrated termination which is thecalibrating shell 10. The pressure PM and the pressure PB are measuredand associated to the J number corresponding to the measure. If J doesnot equal the number of measures N required to calculate the K_(T)average, the next measure is taken which means that J is incrementedby 1. The next measure is preferentially made for a pressure incrementedof 2 hPa, which means that I is incremented by two. When the number ofmeasures required N is reached, the data associated to each measure arecomputed and enable to associate a tube coefficient K_(T) value to eachcorresponding measure J. This enables calculating of an average of theK_(T) value. For example if eight measures are required, J and I will beincremented 7 times before calculating the tube coefficient k value.This also enables rejecting the tube if its standard does not correspondto the kind of tube which can be used with the apparatus 1. The K_(T)value will then be used for the airflow computation. When at least onefilter 22 is placed at a first extremity 13 of tube, the control unit 2is able to calculate the airflow at the second extremity 14 of the tubefrom the measured pressures PM and PB and from the airflow resistancecoefficient K_(T) of said tube and from the airflow resistancecoefficient KF of said filter.

The apparatus as described above, wherein the control unit comprisesoffset compensation means for compensating the possible difference ofgauging between the two pressures.

Another embodiment of the present invention is improving the sensitivityof the airflow measurement at low values, by gauging the pressuresensors.

Low flows accuracy is important in an air delivery apparatus especiallywhen triggering between inspiration and expiration where sensitivitiesas low as 5 l/mn are required.

A classical 22 mm diameter breathing tube will present a pressure drop(PB−PM) of 0.01 hPa with an airflow of 5 l/mn, so it is mandatory toconsider a signal amplification after the subtraction.

But this amplification cannot be done without a prior offsetcompensation. The apparatus according to the preferential implementationenables to correct this difference of gauging.

As previously described, using two pressure sensors at both ends of abreathing circuit tube, can give an accurate value of airflow, but dueto physical constraints, for low flows accuracy, there is a need foramplification.

Due to the manufacturing process of airflow sensors, most of them arepresenting a Voltage vs. Pressure relationship like:Vout=Voffset+Kps·Pr

With

Vout being the output voltage

Voffset being the constant that can change drastically from one sensorto another within the same lot and that drifts slowly due to aging.

Kps: the gain in volts/hPa of the pressure sensor that is usuallystable.

Pr: the difference between pressure sensors 6 and 8

Given the previous equation, subtracting the two voltages of the twopressure sensors will then give:VPbPm=VoffPm−VoffPb+Kps*(Pb−Pm)

With

VPbPm is the Voltage result of the subtraction

VoffPm is the offset of the Pm pressure sensor

VoffPb is the offset of the PB pressure sensor

Kps is the gain in volts/hPa of the pressure sensors

By offset it is meant that the constant which corresponds to thedifference between the pressure measured by one sensor and the absolutevalue of pressure.

The invention takes advantages of a well known state of the apparatuswhen no patient is connected to the apparatus, and no pressure isgenerated by the blower. During this state The PM and PB values have thesame value as the ambient pressure. The control unit 2 comprises offsetcompensation means for compensating the possible difference of gaugingbetween the two pressure sensors 6 and 8.

Preferentially the offset compensation means comprise:

-   -   a digital to analog converter 32 connected to a microprocessor        30 in order to convert microprocessor's digital data in analog        data,    -   an analog substractor 34 having inputs connected to the second        pressure sensor, to the first pressure sensor, and to said        digital to analog converter;        said microprocessor calculating, when the blower is not        functioning, the difference between the two pressures measured        by said first and second pressure sensors and then sending the        value C of this difference to said digital to analog converter,        which converts said value C in analog data and drive it to said        analog subtractor, which subtract the pressure P_(B) measured by        said second pressure sensor and said value C to the pressure        P_(M) measured by said second pressure sensor and send the        corresponding result D to the microprocessor, which will modify        the C value until said D result equals zero, said microprocessor        capturing the C value when said D result equals zero, enabling        the microprocessor which uses the pressure sensors to control        the apparatus to correct the difference of offsets between the        pressure sensors.

Preferentially the apparatus comprises an analog amplifier 36 connectedto said analog subtractors 34 and 38 in order to amplify the signalcorresponding to said D result and to send it to the microprocessor 30,thus enabling the microprocessor to have an accurate adjustment of saidvalue C until the result D reaches the value zero.

The apparatus can also comprise analog to digital converters 42, 44 and40 connected between the microprocessor 30 and the said first pressuresensor, between the microprocessor and the said second pressure sensor,and between the microprocessor and the said analog amplifier, so thatthe microprocessor is provided with only digital data.

With a non standard calibration tube, the process for calibrating a tubeused in the apparatus to assist patient's respiration, comprises:

connecting a first extremity 13 of said tube 20 to the blower of anapparatus,

-   -   if the tube is not provided with a pressure sensor at its second        extremity, placing said second pressure sensor at said second        extremity,    -   connecting said second extremity to a shell 10 with a traversing        hole 12 having a known airflow resistance coefficient K_(s),

switching the blower on, and instructing said control unit 2 to measurethe pressures on both said first and second pressure sensors,

-   -   calculating the value of the tube airflow resistance coefficient        K_(t) from these measured pressures and from the said        coefficient K_(s).

The apparatus can also comprise a Frequency Shift Keying (FSK) modulatorwhich transforms the binary data sent by the apparatus sensors orelements in a modulation of the frequency of the tension applied on avoltage controlled current source, connected to the external powersupply, so that the voltage controlled current source transmit themodulation corresponding to the data, a FSK demodulator converting thevoltage frequency modulation into binary data and transmit to theelements, so that each sensor or module connected to the power source isable to receive or transmit information.

The apparatus can also be used in a set for calibrating a tube used inapparatus to assist patient's respiration comprising the apparatusaccording to present invention and a calibrating shell 10 with atraversing hole 12 having a known airflow resistance coefficient K_(S).

The apparatus enables modulation of the pressure to the patient inrespect to the illness to treat. Due to the airflow computation, theapparatus has the capacity to differentiate the two basic states of therespiration: inspiration and expiration. The control unit comprises anonvolatile memory 120 in which the control unit stores, as two values,the pressures measured at each pressure sensor. The sensors provided inthe apparatus enables the pressure control unit to control the pressureof the air delivered. The outputs of the Estimator are the value of theinspiration pressure PI which is the pressure maintained at thepatient's mask 15 during the inspiration, and the value of theexpiration pressure PE which is the pressure maintained at the patient'smask 15 during the expiration. The data of the pressures PM 112 and PB114 which are sensed at the extremities of the tube and the data 116 ofthe tube coefficient K_(T) enable the airflow computation. Thiscomputation 130 enables the computation of the inspiration andexpiration, this latest computation enables the estimation module 100 todeterminate, which step of the patient's breathing is occurring. Abreath estimation step is qualifying a breath in shape, energy (volume)and frequency. The clinician or a qualified user enters parameters ofthe delivered pressures for the expiration phase and the inspirationphase. The clinician also enters parameters defining how the estimationmodule 100 is going to react following events detected in the breathestimator 132. It is well known that a feedback of the patient with histreatment is helping compliance, thus the patient can have an access toa parameter ranging from min to max that is qualified to be “comfort vs.efficiency”. This patient setting is having the weight that theclinician is giving to it, from pure placebo effect to some level ofeffects. Basically the patient settings 122 are applied in the normalbreath situation or/and have a limited action on the pressureregulation. It is also possible that the airflow is an input to theestimation module 100. Thus, with the data inputs concerning the breathestimation (and clinical symptoms or event associated with), theinspiration/expiration computation and the clinical settings, andpossibly the airflow computation and patient settings, the control unit2 by the estimation module 100 is able to determinate the pressuresrequired PI and PE. Those two values can be addressed to two differentoutputs 102 and 104 where a switch is able, relative to theinspiration/expiration computation, to connect to the required outputregarding if the patient is breathing in or out. The control unit 2comprises a pressure control loop 106 which, by comparing the pressuremeasured in the mask 15 and the value of pressure required PI or PE, isable to adjust the Pulse Width Modulation tension PWM in order to obtainthe correct pressure in the mask 15. The FIG. 6 represents one patternof the pressure of treatment provided according to the airflow due tothe patient breathing. In this example, the clinical technician has seta special modulation of pressures Pi and Pe during respectively theinspiration and the expiration; after a while as no special event occursthe values of the two pressures are changed.

The apparatus has a two steps strong recognition process in order toprevent false start of the apparatus when the mask 15 is not on thepatient's face and to prevent starting a new treatment session. When theapparatus is started by the patient by using the keyboard, for exampleand as represented in FIG. 7 by using the start key, the blower 33 iskept turning at a very low speed, waiting for some activity on the maskpressure sensors 6. When an activity is detected, the apparatus isinstantaneously trying to bring the pressure at the apparatus outlet ata minimum starting pressure SP of 4 hPa. When this pressure is reachedthe apparatus tries to identify at least one breath to start the processaccording to the settings. When the mask 15 is not applied againstsomething, like a hand or the patient's face, no activity is detected.Then if a maximum time since the apparatus start has been spent (thetimeout is reached), the blower is stopped. On the contrary, theapparatus keeps on waiting for an activity on the pressure sensors. Whenthe mask 15 is not applied correctly the pressure can not reach 4 hPa.Then if the timeout is reached the blower is stopped, on the contrarythe apparatus waits to detect some activity on the pressure sensors.When the mask 15 is not applied on the patient's face no breath patternis recognized. Then the blower is stopped if the timeout is reached. Onthe contrary the apparatus waits to detect some activity on the pressuresensors. The timeout checking prevents the blower keeping turning on ifthe patient does not start the treatment and forgets to start theblower. A further advantage of this implementation is that if a patientis connected, bringing the pressure instantaneously to 4 hPa willprevent CO₂ rebreathing.

The following examples demonstrates the way the estimation module 100modulates the pressure value P_(M) to apply to the patient's mask.

EXAMPLE 1 Variations of the Pressure of the Average Pressure ofTreatment as a Function Time According to Detected Events

In a preferential embodiment the average pressure of treatment on onebreathing step is not constant in time and will be modulated by theestimation module 100 according to the events occurring, such as snoringor apneas.

The apparatus will try to reduce the average pressure of treatmentvalue, thus enhancing the patient comfort while breathing against theapparatus. The clinician set a minimum average pressure of treatmentPtmin and a coefficient N_(OEK) expressed in hPa/s.

As represented on FIG. 6 when no events are detected the averagepressure of treatment value will follow the equation:PT(t)=MAX(PT(t−ε)-(NOEK×ε), PT min)

εbeing the sampling time, the MAX function is returning the greatestvalue of its two members. The average pressure value AVP, correspondingto the pressure of treatment PT on one breath thus decrease linearlyuntil it reaches the minimum set by the clinician and stays constantuntil an event occurs. The average pressure is changed each samplingtime which correspond to one single breath (one inspiration and oneconsecutive expiration).

If an event is detected, a 3 steps process is initiated:

-   -   Step 1: the estimation module, looking in the clinical settings        is defining if the event has to affect the average value of        pressure PT.    -   Step 2: If so the estimation module looking in clinical        settings, will define a persistence delay D_(p). In the example        of FIG. 6, a snore is detected at t₁₆, the persistence delay DP        =t₁₇-t₁₆ could be set to 2 minutes.    -   Step 3: A Ek parameter in hPa/s is extracted from clinical        settings and balanced with an eventual ongoing event. The        estimation module will determine the Ek corresponding to the        event which occurs or has occurred and linearly increase the        average pressure of treatment with Ek as slope coefficient. AVP        will then follow the equation:        PT(t)=MIN(PT(t−ε)+(EK×ε), AVP max)        ε being the sampling time, the MIN function is returning the        smallest value of its two members.

During the persistence delay even if no event occurs the estimationmodule will keep on increasing the average pressure of treatment PT.

EXAMPLE 2 d1, d2 Auto Adjustment

On the FIG. 6 d1 and d2 have two different behaviors, according to t2and t4 definitions. t2 is defined when the absolute value of airflowstarts to decrease within the inspiration phase or shows a fixed delayafter t1. t4 is defined when the absolute value of airflow starts todecrease within the expiration phase or shows a fixed delay after t3.

If t2 and t4 are defined according to the airflow waveform then noauto-adjustment is occurring. If not, following the same rules than AVP,the d1 and d2 can also be affected by the events. In the case when d1 &d2 are not locked by the breath waveform following the same process, d1& d2 can also be adjusted according to events in the case.

1. An apparatus to assist a patient's respiration by delivering air tosaid patient through a mask, said mask being designed to be connected ona first extremity of a tube, said apparatus comprising: a control unitto adjust the pressure delivered by a blower of said apparatus, a firstpressure sensor for measuring a first pressure, said first pressuresensor being connected to said control unit, and a second pressuresensor for measuring a second pressure at an air output of said blower,said second pressure sensor being connected to said control unit;wherein when a tube is connected between said apparatus and acalibrating shell with a traversing hole having a known airflowresistance coefficient K_(S), the air flows from the apparatus to saidcalibrating shell, and the measured pressures are sent to said controlunit, which calculates a tube airflow resistance coefficient K_(T) basedon said first and second measured pressures and said known airflowresistance coefficient K_(S), wherein when a tube is connected to saidmask and connected to said apparatus on a second extremity of said tube,the air flows from the apparatus to the mask, and said control unitcalculates the airflow at said second extremity of the tube based onsaid first and second pressures and said airflow resistance coefficientK_(T) of said tube.
 2. The apparatus according to claim 1, wherein thecontrol unit comprises an offset compensation means for compensating apossible difference of gauging between the two pressure sensors.
 3. Theapparatus of claim 1, wherein said first and second pressure sensorssense the pressure at both extremities of said tube.
 4. An apparatus toassist a patient's respiration by delivering air to said patient througha mask, said mask being designed to be connected on a first extremity ofa tube, said apparatus comprising: a control unit to adjust the pressuredelivered by a blower of said apparatus, a first pressure sensor formeasuring a first pressure, said first pressure sensor being connectedto said control unit, and a second pressure sensor for measuring asecond pressure at the air output of said blower, said second pressuresensor being connected to said control unit; wherein when said tube isconnected to said mask and connected to said apparatus on a secondextremity of said tube, the air flows from the apparatus to the mask,and said control unit calculates the airflow at said second extremity ofthe tube based on said first and second pressures and an airflowresistance coefficient K_(T) of said tube; and wherein the control unitcomprises an offset compensation means for compensating the possibledifference of gauging between the two pressure sensors.
 5. The apparatusaccording to claim 4, wherein when said tube is connected between saidapparatus and a calibrating shell with a traversing hole having a knownairflow resistance coefficient K_(S), the air flows from the apparatusto said calibrating shell, the measured first and second pressures aresent to said control unit, which calculates the tube airflow resistancecoefficient K_(T) based on said measured first and second pressures andsaid known airflow resistance coefficient K_(S).
 6. The apparatusaccording to claim 4, wherein said offset compensation means comprises:a microprocessor, a digital to analog converter connected to saidmicroprocessor in order to convert said microprocessor's digital data toanalog data, an analog subtractor having inputs connected to the secondpressure sensor, said first pressure sensor, and said digital analogconverter, wherein when the blower is not functioning, saidmicroprocessor calculates the difference between the first and secondpressures measured by said first and second pressure sensors to obtain avalue C, and then sends the value C of said difference to said digitalto analog converter, which converts said value C to analog data anddrives said value C to said analog subtractor, wherein said subtractorsubtracts the second pressure measured by said second pressure sensorand said value C from the first pressure measured by said first pressuresensor and sends the corresponding result D to the microprocessor,wherein said microprocessor modifies the C value until said result Dequals zero, said microprocessor capturing the C value when said resultD equals zero, enabling the control unit to correct the difference ofoffsets between the pressure sensors.
 7. The apparatus according toclaim 6, further comprising an analog amplifier connected to said analogsubtractor in order to amplify the signal corresponding to said result Dand to send said result D to said microprocessor, thus enabling saidmicroprocessor to have an accurate adjustment of said value C until saidresult D reaches the value zero.
 8. The apparatus according to claim 7,further comprising analog to digital converters connected between themicroprocessor and said first pressure sensor, between themicroprocessor and said second pressure sensor, and between themicroprocessor and said analog amplifier, so that the microprocessor isprovided with only digital data.
 9. The apparatus according to claim 4,wherein when at least one filter is placed at said first or secondextremity of said tube, said control unit calculates the airflow at saidsecond extremity of the tube based on said measured first and secondpressures, the airflow resistance coefficient K_(T) of said tube, and anairflow resistance coefficient K_(F) of said filter.
 10. The apparatusaccording to claim 4, wherein said control unit comprises a nonvolatilememory in which the control unit stores two values corresponding to saidfirst and second pressures measured at each of said first and secondpressure sensors, when said control unit forces the blower to deliver adetermined constant pressure I at one of the two sensors, so that whenat least two values corresponding to two different said determinedconstant pressures I are stored, the control unit calculates an averageof said airflow resistance coefficient K_(T).
 11. The apparatusaccording to claim 4, wherein said control unit comprises an estimationmodule connected to a means for detecting the patient's breathingparameters, such that the estimation module determines when the patientis inspiring or expiring, and in response determines the pressure toapply to the patient's mask, so that the control unit adjusts thepressure delivered by the blower.
 12. The apparatus according to claim11, wherein the control unit comprises a nonvolatile memory in which aclinician can enter clinical settings comprising at least the treatmentpressure and possibly the pressure to apply according to the patient'sbreathing parameters, said estimation module applying the pressureaccording to these clinical settings and to the patient's breathingparameters.
 13. The apparatus according to claim 12, wherein the patientcan enter patient settings in said nonvolatile memory, said estimationmodule applying the pressure according to said patient settings and tothe patient's breathing parameters within bounds given by the cliniciansettings.
 14. The apparatus of claim 11, in which the estimation moduleis able to determine that an event (E₁, E₂ or E₃) occurs in saidpatient's breathing, thus enabling said control unit to adjust thetension to apply to the blower to adjust the pressure at said patient'smask.
 15. The apparatus of claim 11, wherein said means for detectingthe patient's breathing parameters enable the control unit to computethe airflow at said patient's mask, said estimation module determiningthat an event (E₁, E₂ or E₃) is occurring with the airflow parameters orshape.
 16. The apparatus according to claim 11, wherein said estimationmodule has an inspiration output where said estimation module sets thefirst pressure value during inspiration and wherein said estimationmodule has an expiration output, and wherein said estimation module setsthe first pressure value during expiration, said control unit comprisinga switch which is connected alternatively to the inspiration output orexpiration output according to the patient's breathing.
 17. Theapparatus according to claim 11, wherein the apparatus further comprisesa starting means which when actuated enables the estimation module todetermine if a breathing activity is detected, the estimation modulesending an instruction to stop the blower if no activity is sensed aftera given delay.
 18. The apparatus of claim 4, further comprising: aFrequency Shift Keying (FSK) modulator which transforms the binary datasent by the apparatus sensors or elements in a modulation of thefrequency of the tension applied on a voltage controlled current source,connected to an external power supply, so that the voltage controlledcurrent source transmits the modulation corresponding to the data, and aFSK demodulator which converts the voltage frequency modulation intobinary data and transmits it to the elements, so that each sensor ormodule connected to the power source is able to receive or transmitinformation.
 19. A kit for calibrating a tube used in apparatus toassist said patient's respiration comprising: the apparatus according toclaim 4, and a calibrating shell with a traversing hole having a knownairflow resistance coefficient K_(S).
 20. A process for calibrating saidtube used in said apparatus to assist a patient's respiration by usingthe apparatus according to claim 4, said process comprising the stepsof: connecting said second extremity of said tube to the blower of saidapparatus, connecting said first extremity to a calibrating shell with atraversing hole having a known airflow resistance coefficient K_(S),connecting said first pressure sensor to measure the first pressure atsaid first extremity of said tube, switching the blower on, instructingsaid control unit to measure the first and second pressures at saidfirst pressure sensor and said second pressure sensor, said secondpressure being measured at an outlet of said blower, and calculating thevalue of the tube airflow resistance coefficient K_(T) based on saidmeasured first and second pressures and said known airflow resistancecoefficient K_(S).
 21. A process for calibrating the tube used inapparatus to assist said patient's respiration, and for calibrating thetube by using the apparatus according to claim 4, said processcomprising the steps of: (1) connecting said second extremity of saidtube to the blower of said apparatus, (2) connecting said firstextremity of said tube to a calibrating shell with a traversing holehaving a known airflow resistance coefficient K_(S), (3) connecting saidfirst pressure sensor to measure the first pressure at said firstextremity of said tube, (4) switching the blower on, (5) fixing at avalue I the pressure provided and measured on one of said two pressuresensors, (6) instructing said control unit to measure the pressures atsaid first pressure sensor and second pressure sensor, said secondpressure being measured at an outlet of said blower, (7) storing saidfirst and second pressures as a measured pressure values associated withsaid value I, (8) repeating steps 5 and 6 a number of times N, saidvalue I being different for each time, so that each of said measuredpressure values are associated with one value I, and (9) calculating onaverage of the airflow resistance coefficient K_(T) based on said firstand second measured pressures and said known airflow resistancecoefficient K_(S).
 22. A process for assisting a patient's respirationby delivering air to the patient through a mask, comprising the stepsof: providing an apparatus comprising a control unit for adjusting thepressure delivered by a blower of said apparatus, a first pressuresensor and a second pressure sensor, said second pressure sensor beingprovided at an air outlet of said blower, said first and second pressuresensors being connected to said control unit; providing a tube, acalibrating shell and said mask; calibrating said tube by connecting afirst extremity of said tube to said calibrating shell with a traversinghole having a known airflow resistance coefficient K_(S) and connectinga second extremity of said tube to the air outlet of said blower;switching said blower on, the air flowing from said apparatus to saidcalibrating shell; measuring a first pressure at said first pressuresensor and a second pressure at said second pressure sensor; sending thefirst and second measured pressures to said control unit, wherein saidcontrol unit calculates a tube airflow resistance coefficient K_(T)based on said first and second measured pressures and said known airflowresistance coefficient K_(S); connecting said first extremity of saidtube to said mask and applying said mask on the patient's face;switching the blower on, the air flowing from said apparatus to saidmask; and measuring a first pressure at said first pressure sensor and asecond pressure at said second pressure sensor, wherein said controlunit calculates the airflow at said second extremity of said tube basedon the first and second pressures and the airflow resistance coefficientK_(T) of said tube.
 23. The process according to claim 22, furthercomprising the step of: compensating a possible difference of gaugingbetween said two pressure sensors using an offset compensation means,wherein said control unit further comprises said offset compensationmeans.
 24. The process of claim 22, wherein the first and secondpressure sensors are sensing the pressure on both extremities of thetube.
 25. A process for assisting a patient's respiration by deliveringair to said patient through a mask, comprising the steps of: providingan apparatus comprising a control unit comprising an offset compensationmeans for adjusting the pressure delivered by a blower of saidapparatus, and first and second pressure sensors connected to saidcontrol unit; providing a tube and said mask; connecting a firstextremity of said tube to said mask, connecting a second extremity ofsaid tube to said apparatus on a second extremity of said tube, andapplying said mask to the patient's face; switching said blower on, theair flowing from said apparatus to said mask, wherein said control unitcalculates the airflow at said second extremity of the tube based onfirst and second pressures and an airflow resistance coefficient K_(T)of said tube; and compensating a possible difference of gauging betweensaid two pressure sensors using said offset compensation means.
 26. Theprocess according to claim 25, further comprising the steps of:calibrating said tube by connecting said first extremity of said tube toa calibrating shell with a traversing hole having a known airflowresistance coefficient K_(S) and connecting said second extremity ofsaid tube to said apparatus; switching said blower on, the air flowingfrom said apparatus to said calibrating shell; measuring the firstpressure at said first pressure sensor and the second pressure at saidsecond pressure sensor; and sending the first and second measuredpressures to said control unit, wherein the control unit calculates thetube airflow resistance coefficient K_(T) based on said measured firstand second pressures and said known airflow resistance coefficientK_(S).
 27. The process according to claim 25, wherein said offsetcompensation mean further comprises a microprocessor, a digital toanalog converter connected to said microprocessor in order to convertsaid microprocessor's digital data to analog data, and an analogsubtractor having inputs connected to the second pressure sensor, saidfirst pressure sensor, and said digital analog converter, the processfurther comprising the steps of: calculating, when said blower is notfunctioning, the difference between the first and second pressuresmeasured by said first and second pressure sensors with saidmicroprocessor to obtain a value C, and sending the value C of thedifference to said digital to analog converter, which converts the valueC to analog data and drives the value C to said analog subtractor,subtracting the second pressure measured by said second pressure sensorand the value C from the first pressure measured by said first pressuresensor with said subtractor and sending the corresponding result D tosaid microprocessor, and modifying the C value until the result D equalszero with said microprocessor, said microprocessor capturing the C valuewhen said result D equals zero, enabling said control unit to correctthe difference of offsets between said first and second pressuresensors.
 28. The process according to claim 27, further comprising thesteps of: amplifying the signal corresponding to the result D with ananalog amplifier connected to said analog subtractor, and sending theresult D to said microprocessor, thus enabling said microprocessor tohave an accurate adjustment of the value C until the result D reachesthe value zero.
 29. The process according to claim 28, wherein saidanalog to digital converters are connected between said microprocessorand said first pressure sensor, between said microprocessor and saidsecond pressure sensor, and between said microprocessor and said analogamplifier, so that said microprocessor is provided with only digitaldata.
 30. The process according to claim 25, further comprising the stepof calculating the airflow at said second extremity of the tube based onsaid measured first and second pressures, the airflow resistancecoefficient K_(T) of said tube, and an airflow resistance coefficientK_(F) of said filter with said control unit, wherein at least one filteris placed at the first or second extremity of said tube.
 31. The processaccording to claim 25, further comprising the steps of: storing twovalues corresponding to the first and second pressures measured at eachof said first and second pressure sensors in a non-volatile memory insaid control unit when said control unit forces said blower to deliver adetermined constant pressure I at one of said sensors, and calculatingan average of said airflow resistance coefficient K_(T) when at leasttwo values corresponding to two different said determined constantpressures I are stored.
 32. The process according to claim 25, furthercomprising the steps of: determining when the patient is inspiring orexpiring with an estimation module connected to a means for detectingthe patient's breathing parameters, determining the pressure to apply tothe patient's mask, and in response, adjusting the pressure delivered bysaid blower.
 33. The process according to claim 32, further comprisingthe steps of: entering into a non-volatile memory in the control unitclinical settings comprising at least the treatment pressure andoptionally the pressure to apply according to the patient's breathingparameters, said estimation module applying the pressure deliveredaccording to these clinical settings and to the patient's breathingparameters.
 34. The process according to claim 33, wherein the patientcan enter patient settings in said nonvolatile memory, said estimationmodule applying the pressure according to said patient settings and tothe patient's breathing parameters within bounds given by the cliniciansettings.
 35. The process of claim 32, further comprising the steps of:determining that an event (E1, E2 or E3) occurs in said patient'sbreathing with said estimation module, and adjusting the tension toapply to said blower to adjust the pressure delivered to said patient'smask with said control unit.
 36. The process of claim 32, furthercomprising the steps of: computing the airflow with said control unit atsaid patient's mask based on airflow parameters received from a meansfor detecting the patient's breathing parameters, and determining thatan event (E1, E2 or E3) is occurring based on the airflow parameterswith said estimation module.
 37. The process according to claim 32,further comprising the step of setting the first pressure value duringinspiration and providing said estimation module with an expirationoutput with said estimation module, and setting the first pressure valueduring expiration with said estimation module, wherein said control unitfurther comprises a switch which is connected alternatively to theinspiration output or expiration output according to the patient'sbreathing.
 38. The process according to claim 32, further comprising thesteps of: determining if a breathing activity is detected with astarting means when said starting means is actuated, and sending aninstruction to stop said blower from said estimation module to saidblower if no activity is sensed after a given delay.
 39. The process ofclaim 25, further comprising the steps of: transforming the binary datasent by the apparatus sensors or elements with a Frequency Shift Keying(FSK) modulator into a modulation of the frequency of the tensionapplied on a voltage controlled current source, connected to an externalpower supply, transmitting the modulation corresponding to the data fromthe voltage controlled current source, converting the voltage frequencymodulation with an FSK demodulator into binary data, transmitting thevoltage frequency modulation to the apparatus sensors or elements, suchthat each sensor or module connected to the power source receives ortransmits information.
 40. The process of claim 25, further comprisingthe steps of: providing a kit comprising said apparatus, a calibratingtube and a calibrating shell with a traversing hole having a knownairflow resistance coefficient K_(S).
 41. The process of claim 25,further comprising the steps of: calibrating the tube by: connectingsaid second extremity of said tube to said blower of said apparatus,connecting said first extremity to a calibrating shell with a traversinghole having a known airflow resistance coefficient K_(S), connectingsaid first pressure sensor to measure the first pressure at said firstextremity of said tube, switching the blower on, instructing saidcontrol unit to measure the first and second pressures at said firstpressure sensor and said second pressure sensor, said second pressurebeing measured at an outlet of said blower, and calculating the value ofthe tube airflow resistance coefficient K_(T) based on said measuredfirst and second pressures and said known airflow resistance coefficientK_(S).